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Some possibilities (in approximate order) are: 1) intake manifold gasket(s), 2) head gasket(s), 3) cracked head(s), 4) cracked block, 5) head bolts, or 6) cracked intake manifold. The thing to do is to remove all of the spark plugs, top off and then pressurize the cooling system for several minutes using a radiator pressure tester. Next, have an assistant crank the engine a couple of complete revolutions while you watch for a cloud of coolant fog blowing out of a cylinder (don't stand in line with any spark plug holes). This will tell you whether coolant is leaking into a cylinder and which one. Otherwise, you may be able to narrow the source of the leak by careful observation after pressurizing the radiator.
Intake gaskets have a tendency to deteriorate at the four corners of the intake manifold adjacent to the cylinder head coolant ports. Head gasket failures are more often a result of combustion gasses blowing past the fire ring into the cooling system, resulting in engine overheating and white exhaust smoke accompanied by a sweet exhaust odor. Cracks are more difficult to predict and diagnose without engine teardown. Rarely, head bolts will leak coolant past their threads into the rocker house - more often this is a result of improper engine assembly or very poor long-term engine maintenance.
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